1. Field of the Invention
The present invention relates to a display device and a method of fabricating a display device, and more particularly to an organic electroluminescent display device and a method of fabricating an organic electroluminescent display device.
2. Discussion of the Related Art
In general, an organic electroluminescent display (OELD) device emits light by injecting electrons from a cathode and holes from an anode into an emission layer, combining the electrons and the holes, generating an exciton, and transitioning the exciton from an excited state to a ground state. Contrary to a liquid crystal display (LCD) device, an additional light source is not necessary for the OELD devices to emit light because the transition of the exciton between states causes light to be emitted from the emission layer. Accordingly, size and weight of the OELD devices can be reduced. Since the OELD devices have low power consumption, superior brightness, and fast response time, the OELD devices are being incorporated in consumer electronic products, such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabrication of the OELD devices is simple, it is much cheaper to produce OELD devices than LCD devices.
OELD devices may be categorized into passive matrix OELD devices and active OELD matrix devices. Although the passive matrix OELD devices have a simple structure and are formed using simple fabricating processes, the passive matrix OELD devices require relatively high amounts of power to operate and display sizes of the passive matrix OELD devices are limited by their structures. In addition, as a total number of conductive lines increases, aperture ratios of the passive matrix OELD devices decrease. In contrast, the active matrix OELD devices have a high emission efficiency and can produce high-quality images for large displays using relatively low power.
FIG. 1 is a schematic cross sectional view of an OELD device according to the related art. In FIG. 1, an array unit 14 including a thin film transistor (TFT) “T” is formed on a first substrate 12. A first electrode 16, an organic electroluminescent layer 18, and a second electrode 20 are sequentially formed on the array unit 14, wherein the organic electroluminescent layer 18 may separately display red, green, and blue colors for each pixel region. In general, separate organic materials are commonly used to emit light of each color for the organic electroluminescent layer 18 in each pixel region. An organic ELD device is encapsulated by attaching the first substrate 12 and a second substrate 28, which includes a moisture absorbent material 22, with a sealant 26. The moisture absorbent material 22 eliminates any moisture and oxygen that may penetrate into a capsule of the organic electroluminescent layer 18. After etching a portion of the second substrate 28, the etched portion is filled with the moisture absorbent material 22 and the filled moisture absorbent material is fixed by a holding element 25.
FIG. 2 is a schematic plan view of an array unit of an OELD device according to the related art. In FIG. 2, an array unit of an OELD device includes a switching element “TS,” a driving element “TD,” and a storage capacitor “CST,” wherein the switching element “TS” and the driving element “TD” may include a combination of at least one thin film transistor (TFT). A transparent insulating substrate 12 upon which the array unit is formed may be made of glass or plastic material. A gate line 32 and a data line 34 crossing each other are formed on the substrate 12, wherein a pixel region “P” is defined by the crossing of the gate line 32 and the data line 34. An insulating layer (not shown) is interposed between the gate line 32 and the data line 34, and a power line 35 parallel to and spaced apart from the data line 34 crosses the gate line 32.
The switching element “TS” is a thin film transistor including a switching gate electrode 36, a switching active layer 40, and a switching source and drain electrodes 46 and 50. Similarly, a driving element “TD” is a thin film transistor including a driving gate electrode 38, a driving active layer 42, and a driving source and drain electrodes 48 and 52. The switching gate electrode 36 is connected to the gate line 32 and the switching source electrode 46 is connected to the data line 34, and the switching drain electrode 50 is connected to the driving gate electrode 38 through a first contact hole 54. The driving source electrode 48 is connected to the power line 35 through a second contact hole 56. In addition, the driving drain electrode 52 is connected to a first electrode 16 at the pixel region “P.” The power line 35 overlaps a first capacitor electrode 15 with the insulating layer interposed therebetween to form the storage capacitor “CST.”
FIG. 3 is a schematic plan view of an OELD device according to the related art. In FIG. 3, a substrate 12 includes a data pad region “E” at a first side and first and second gate pad regions “F1” and “F2” at second and third sides adjacent to the first side. A common electrode 39 is formed at a fourth side facing the first side and adjacent to the second and third sides of the substrate 12, wherein a common voltage is applied to a second electrode 20 through the common electrode 39 to maintain an electrical potential of the second electrode 20. Accordingly, a display region at a center of the substrate 12 is used for displaying images.
FIG. 4A is a schematic cross sectional view along IVa—IVa of FIG. 2 according to the related art, and FIG. 4B is a schematic cross sectional view along IVb—IVb of FIG. 3 according to the related art. In FIGS. 4A and 4B, a driving thin film transistor (TFT) “TD” including a driving active layer 42, a driving gate electrode 38, and driving source and drain electrodes 56 and 52 are formed on a substrate 12. An insulating layer 57 is formed on the driving TFT “TD” and a first electrode 16 connected to the driving drain electrode 52 is formed on the insulating layer 57. An organic emission layer 18 for emitting light of a specific color is formed on the first electrode 16 and a second electrode 20 is formed on the organic emission layer 18. A storage capacitor “CST” is formed to be electrically parallel to the driving TFT “TD” and includes first and second capacitor electrodes 15 and 35, wherein a portion of a power line overlapping the first capacitor electrode 15 is used as the second capacitor electrode 35, the second capacitor electrode 35 is connected to the driving source electrode 56, and the second electrode 20 is formed on an entire surface of the substrate 12 including the driving TFT “TD,” the storage capacitor “CST,” and the organic emission layer 18.
A common electrode 39 through which a common voltage is applied to the second electrode 20 is formed at a peripheral portion of the substrate 12, wherein the common electrode 39 is simultaneously formed with the driving source and drain electrodes 56 and 52. Multiple insulating layers on the common electrode 39 include first and second contact holes 50 and 52 exposing the common electrode 39, and the second electrode 20 is connected to the common electrode 39 through the first contact hole 50. Although not shown, an external circuit is connected to the common electrode 39 through the second contact hole 52 to supply the common voltage.
However, when an array unit and an emission unit are formed on one substrate, production yield of an OELD device is determined by a multiplication of TFT yield and organic emission layer yield. Since the organic emission layer yield is relatively low, the production yield of an ELD is limited by the organic layer yield. For example, even when a TFT is fabricated, an OELD device can be determined to be bad due to defects of an organic emission layer. Accordingly, production materials are lost and production costs rise.
In general, OELD devices are classified into bottom emission-type OELD devices and top emission-type OELD devices according to an emission direction of light used for displaying images. Although bottom emission-type OELD devices have advantages of high encapsulation stability and high process flexibility, they are ineffective for high resolution devices because they have poor aperture ratios. In contrast, top emission-type OELD devices have higher expected life spans because they are easily designed and have high aperture ratios. However, in top emission-type OELD devices, the cathode is commonly formed on an organic emission layer, wherein transmittance and optical efficiency are reduced since a limited number of materials may be selected. When a thin film protection layer is used to minimize the transmittance reduction, the top emission-type OELD devices are not sufficiently shielded from ambient air.